It is well known to use SAW devices for filtering purposes in microwave radio communications systems. With increasing capacities of such systems, very stringent demands have been placed on the design and fabrication of such SAW devices, so that they have been developed to an advanced stage. For example, Suthers et al. U.S. Pat. No. 4,814,658 issued Mar. 21, 1989 and entitled "SAW Device with Continuous Finger Pattern" describes and claims an advanced form of SAW device which is particularly useful for filtering purposes in microwave radio systems, and discusses earlier developments embodied in such a SAW device.
In addition to filters, microwave radio systems include other circuits, such as equalizers, which can benefit by the incorporation therein of appropriate SAW devices. In particular, a microwave radio receiver invariably includes an ATDE (adaptive time domain equalizer), which may incorporate a transversal filter comprising a tapped delay line. The U.S. patent application Ser. No. 382,921 referred to above describes and claims a SAW device tapped delay line which is particularly suitable for use at the IF (intermediate frequency) of a high capacity microwave radio communications system.
A problem arises with such SAW device tapped delay lines in that the transmitted symbol rate of the system and the IF are independently determined. In particular, the IF is not an integral multiple of half the symbol rate. The symbol rate determines the pitch, or center-to-center spacing, of the IDTs (inter-digital transducers) which form the taps of the tapped delay line, whereas the pitch of the inter-digital fingers themselves is determined by the IF. Consequently, the IDT pitch is not an integral multiple of the finger pitch, so that there is necessarily a discontinuity in the finger pitch between adjacent IDTs. Such discontinuities give rise to reflections of a propagated SAW among the IDTs, with consequent deterioration in performance, and this is exacerbated by the large number (for example, 17) of IDT taps which may be necessary to achieve the required level of performance in a high capacity communications system.
In the SAW device tapped delay line of the above application, various arrangements of grounded dummy fingers are provided between adjacent IDT taps, but in each instance there is a discontinuity in periodicity due to the above consideration. In addition to the above application, there are various arrangements of SAW device tapped delay line described in the prior art. However, again in each of these known arrangements there is a discontinuity in finger periodicity from each IDT tap to the next, usually no fingers at all being present in regions between the IDTs. As described above, these discontinuities give rise to impaired performance, which is sufficient largely to prevent SAW device tapped delay lines from being implemented in practical communications systems, in spite of the considerable advantages which they could otherwise provide.
An object of this invention, therefore, is to provide an improved SAW device tapped delay line in which the problem described above is substantially eliminated.